To quantify the impact of organophosphate pesticides on aquatic ecosystems requires a mechanistic understanding of their behaviour in a range of environmental matrices. The objective of this study was to compare the sorption/desorption, biodegradation and toxic effects of the Pestanal grade and commercial formulation (Ectomort Centenary) of the organophosphate insecticide propetamphos in river and estuarine sediments. For both formulations, the sorption of propetamphos onto sediment was initially very rapid followed by a slower sorption phase. Similarly, the initial rate of desorption was rapid, followed by a much slower rate. In both sorption and desorption experiments, the level of sorbed propetamphos was considerably higher for the commercial formulation of propetamphos (Kd=7-11) than for the Pestanal grade (Kd=4-10). The rate of propetamphos biodegradation was sediment dependent but was most rapid where microbial activity and nutrients were the highest and sorption was the lowest. Propetamphos was more rapidly degraded in sediments under aerobic (t(1/2)=15 d) compared to anaerobic conditions (t(1/2)=19 d). However, no significant difference in the biodegradation rates of the Pestanal grade and commercial formulations of propetamphos were observed. The toxic effect of propetamphos on sediment microbial communities was significantly greater for the commercial formulation than for the Pestanal grade of propetamphos based on EC50 (21 versus 236 microg g(-1)) and EC10 values (0.3 versus 54 microg g(-1)). In conclusion, our results highlight the importance of using commercial pesticide formulations when carrying out ecotoxicological testing.